The present invention relates to an instrument system, including an electron microscope, for use in observation, analysis, and evaluation in the course of research/development and manufacture of electronic devices and micro-devices, such as a semiconductor device, liquid crystal device, and magnetic head.
In the case of manufacturing devices, such as a semiconductor memory, there are situations where foreign particles generated in the course of the manufacturing process are mixed therein. Examples of the foreign particles include foreign species particles attributable to process material represented by the residue of etching operations and the residue of a resist, the wall material of process vessels, the material for fixedly holding a wafer, and the material used for a vacuum gas line, etc. Adhesion of such foreign particles to a wafer results in the generation of defective items at times.
It is important from the viewpoint of improving the yield in the manufacture of various devices to analyze the respective elemental composition of the foreign particles that have adhered to a wafer, and to search for the sources of the foreign particles on the basis of their kinds, thereby removing the causes of generation thereof.
As means for obtaining information on the elemental compositions of specimens, there is a known technique of irradiating the specimen with an electron beam, thereby detecting X-rays that are generated. The X-rays comprise a characteristic X-ray emitted when electrons of atoms on the surface of, and in the vicinity of the surface of, specimens fall from an excited state into a lower energy state, and a continuous X-ray at an energy level below the energy of an incident electron beam due to braking radiation, whereby incident electrons are braked before emission. The characteristic X-ray has energy inherent to respective elements, indicated by K, L, and M lines, respectively, depending on the excited state of the characteristic X-ray. Accordingly, the elemental composition of specimens can be found by analyzing the energy at peaks appearing in a spectrum. This method is called energy dispersive X-ray spectroscopy (EDX or EDS). Instruments for performing this method, supplied by companies such as Oxford Instrument, EDAX, TermoNORAN Instrument, and so forth, are available on the market, and they are capable of providing both qualitative analysis and quantitative analysis. Users can find the elemental composition of specimens by analyzing obtained spectra by means of qualitative analysis and quantitative analysis, respectively.
Another example of a method of identifying the elemental composition of specimens from X-ray spectra is disclosed in JP-A No. 108253/1988 (example 1). This publication describes a method in which respective characteristic X-ray spectra (reference spectra) of a plurality of known substances are registered in a memory, and by checking the X-ray spectrum of an unknown substance against the reference spectra registered in the memory, the unknown substance is identified.
An example of the inspecting of foreign particles on the surface of a wafer by use of the method described is disclosed in JP-A No.14811/1996 (example 2). In this example, there is a configuration wherein the locations of foreign particles are determined by observation of images dependent on the magnitude of reflection electron signals, and, by checking the X-ray spectra of the foreign particles against reference spectra, the elemental compositions of the foreign particles can be identified.
Still another method is disclosed in JP-A No. 321225/2000 (example 3). This publication describes a method wherein the net X-ray spectrum of a foreign particle is found on the basis of an X-ray spectrum of a portion of the surface of a wafer having the foreign particle, and an X-ray spectrum of the rest of the surface of the wafer having no foreign particle, (background spectrum), and the elemental composition of the foreign particle is found by checking the net X-ray spectrum of the foreign particle against a database.
Further, JP-A No. 68518/2001 discloses a method of generalizing information on foreign particles, found by the method described above, and registering the same into predetermined categories, thereby specifying causes of defects.
An electron beam, even if focused in a narrow region, is subjected to interaction with the substance inside a specimen upon impacting on the specimen, thereby undergoing scattering. The magnitude of a scattering region is dependent on the element which serves as the constituent of the specimen and the acceleration voltage of the electron beam. FIGS. 18A through 18D are views of the results of a calculation using a Monte Carlo method, showing electron beam scattering conditions when electron beams with acceleration voltage at 15 kV and 5 kV, respectively, are irradiated to specimens of silicon (Si) and tungsten (W), respectively. In the case of the specimen being silicon, the magnitude of a scattering region of the electron beam is about 4 xcexcm, if the acceleration voltage is 15 kV, and it is about 0.4 xcexcm, if the acceleration voltage is 5 kV. Due to the excitation of the electron beams, X-rays are generated substantially in these regions, respectively. This means that the X-ray spectra as observed reflect information on not only the irradiation points of the electron beams, but also the substances contained in the respective scattering regions. Accordingly, the space resolving power in elemental analysis is determined not by the size of an electron beam, but by the magnitude of the scattering region.
Since the processing sizes of semiconductor elements that have attained miniaturization have lately reached sub-micron levels, the sizes of foreign particles causing degradation in the characteristics of the elements have also become smaller. FIG. 19 is a view showing a semiconductor device structure as it appears during a manufacturing process, having respective scattering regions of the electron beams, inside the Si, as shown in FIGS. 18A and 18B. In the case of EDX analysis of a small foreign particle, an electron beam passes through the foreign particle and scatters inside the substrate. Accordingly, an X-ray spectrum as observed contains information on both the foreign particle and the substrate (background), causing difficulty with the analysis. For a substrate in the middle of a manufacturing process, in particular, patterns, that is, an oxide film, electrodes, a dielectric film, and so forth, are formed on the substrate; and, in a case where flakes from those substances constitute foreign particles, the foreign particles need to be distinguished from those substances.
Further, if the acceleration voltage is lowered in order to reduce the effect of the background, that is, to reduce the size of the scattering region, the characteristic X-rays that can be excited are restricted, in which case, elements need to be identified with overlapping characteristic X-ray peaks. Such an instance will be described with reference to FIG. 20. FIG. 20 is a graph showing X-ray spectra of a titanium (Ti) foreign particle 50 nm thick, that is present on the surface of a silicon wafer. The X-ray spectra were obtained by two electron beams having an acceleration voltage of 15 kV and 5 kV, respectively. In the case of the acceleration voltage at 15 kV, a Ti-K line peak is observed at 4.51 keV of X-ray energy; however, in the case of the acceleration voltage at 5 kV, such a peak is not observed because such a characteristic X-ray cannot be excited. In this case, the presence of a titanium element is determined by a Ti-L line that is observed at 0.45 keV of X-ray energy. However, since there exist K-line peaks of oxygen and nitrogen, respectively, in this region of X-ray energy, the characteristic X-ray peaks are observed in an overlapped state, if those elements are present, causing difficulty with the analysis.
Further, in the case of lowering the acceleration voltage, the quantity of X-rays being generated decreases, although the current is sufficient for observation of secondary electron images. For example, in the above-described instance, if the setting of the acceleration voltage only is changed from 15 kV to 5 kV, the quantity of X-rays having the Ti-L line peak is one tenth of the quantity of X-rays having the Ti-K line peak, at the acceleration voltage of 15 kV, which is mainly used for identification of the Ti element, thereby causing a problem of degradation of the accuracy in the identification of elements.
Furthermore, the following problems have been encountered with the conventional methods described in the foregoing.
Analysis by software for qualitative analysis and quantitative analysis, for use with X-ray detectors available in the market, relies on a manual, which is too complicated to be used by a lay person, and is insufficient for controlling the process steps, so that there has been a demand for a system that is capable of automatically outputting an elemental composition.
The methods according to the above-referenced examples 1 and 2 are effective in the case of specimens of a uniform elemental composition, however, in the case where there is a small foreign particle on a substrate, there has been a problem in that, even with foreign particles of an identical elemental composition, the X-ray spectra thereof largely differ from each other, depending on the size (thickness) of the foreign particles, as shown in FIGS. 21A and 21B, resulting in a failure to obtain a match with the reference spectra stored in the database. Although it is conceivable to prepare X-ray spectra corresponding to various thicknesses of foreign particles, this has caused a problem of requiring a longer checking time because of the increase in the number of reference spectra. Further, since the sensitivity (spectral sensitivity) against X-ray energy generally varies on a case-by-case basis due to variations in performance of X-ray detectors and the difference between optical systems for detection, there is a need for preparing X-ray spectra to be provided as a database for every instrument. Furthermore, the spectral sensitivity undergoes a change over time due to stains etc. on an X-ray window, at times causing a problem with the checking of an X-ray spectrum.
The method according to the above-referenced example 3 is a method wherein the net X-ray spectrum of a foreign particle is found on the basis of the X-ray spectrum of the portion of the surface of the wafer, having the foreign particle, and the X-ray spectrum of the rest of the surface of the wafer, having no foreign particle, (the background spectrum), and the elemental composition of the foreign particle is found by checking the net X-ray spectrum of the foreign particle against the data base. In this case, there has been encountered a problem in that there is a possibility that erroneous results will be obtained because components of the X-ray spectrum from the background vary depending on the size of the foreign particle, that is, this is not a case of a simple linear sum of the background spectrum and the X-ray spectrum of the foreign particle portion.
The above-mentioned problem will be described by way of example with reference to FIG. 22. The left part of FIG. 22 is a schematic representation showing a case where an electron beam 8 is irradiated to a silicon wafer 20 incorporating body structures 70 made of an element A. The electron beam 8 scatters in a region 71 that is hemispherical in shape inside the silicon wafer 20; and, in the case where the body structures 70 are present within the region 71, an X-ray spectrum, as observed, comprises the characteristic X-ray peak of silicon and that of the element A. Meanwhile, the right hand part of FIG. 22 is a schematic representation showing a case where a foreign particle 22 made of the element A is present on the surface of the same silicon wafer. The scattering region of an electron beam 8 will be a region 71 that is smaller than the region shown in the left hand figure due to the presence of the foreign particle 22. An X-ray spectrum, as observed in this case, also has the characteristic X-ray peak of silicon and that of the element A. There are cases where the X-ray spectrum obtained in the case of the right hand Figure becomes substantially the same as that in the case of the left hand figure, in which case there has occurred a problem in that the peaks will disappear upon subtracting the X-ray spectrum obtained in the case of the right figure from the X-ray spectrum obtained in the case of the left figure, representing the background.
In view of the problems described above, it is an object of the invention to provide an electron microscope, including an apparatus for effecting x-ray analysis, that is capable of analyzing the elemental composition of foreign particles on the surface of a specimen with high space resolving power, high precision, and high throughput, and a method of analyzing specimens using the same.
The foregoing object of the invention can be achieved by adoption of the following features:
(1) An electron microscope according to the invention is characterized in that the current quantity of an electron beam is controlled such that the count-number of X rays from the specimens falls within a range of 1000 to 2000 counts per second.
The electron microscope having an electron beam optical system provided with an electron source and a lens for focusing an electron beam, an optical system controller for controlling the electron beam optical system, a specimen stage on which specimens are to be placed, an electron detector for detecting electrons emitted from the specimens by irradiating the specimens with the electron beam, an X-ray detector for detecting X rays radiated from the specimens, and a processor for processing signals from both the detectors and performing image formation and elemental analysis of the specimens, comprises means for detecting the count-number of X rays per unit time by detecting the X rays with the X-ray detector and for feedback-controlling the current quantity of the electron beam on the basis of the count-number of X rays per unit time. Further, the current quantity of the electron beam is feedback-controlled such that the count-number of X rays from the specimens falls within the range of 1000 to 2000 counts per second.
As a result, the invention can provide an electron microscope that is capable of producing a large quantity of generated X rays without the need for a user to manually adjust the beam current, and without a risk of impairing the performance of the X-ray detector.
(2) The electron microscope according to the invention may further comprise a database having data including X-ray spectra (reference spectra) of a plurality of kinds of standard substances and labels containing names of substances corresponding to the respective reference spectra, and means for performing the steps of:
checking an X-ray spectrum (sample spectrum) of the specimens against the reference spectra in the database;
calculating the degree of matching in spectral shape between the sample spectrum and the reference spectra;
extracting a reference spectrum having the highest degree of matching from the database;
setting up a plurality of X-ray energy regions so as to have sensitivity data for X-ray energy of the X-ray detector, and to include peaks of the sample spectrum when analyzing by identifying substances of the specimens on the basis of the label corresponding to the reference spectrum that is extracted;
standardizing the intensity of the reference spectra into an intensity of the sample spectrum for each of the X-ray energy regions as set up after multiplying the reference spectra by the sensitivity data;
checking the sample spectrum against the reference spectra as standardized and extracting one or a plurality of the reference spectra in descending order of the degree of matching between the sample spectrum and the reference spectra for each of the X-ray energy regions; and
outputting a label or labels corresponding to the one or the plurality of the reference spectra, the degree of matching, and a numerical value used in the standardization.
Further, a function of outputting the label, the degree of matching, and the numerical value as described above may include, for example, outputting first to third candidate elements in descending order of the degree of matching. Still further, the electron microscope according to the invention may display an intensity ratio of the sample spectra obtained by electron beam irradiation at not less than two varied acceleration voltages. Furthermore, the sensitivity data may contain an intensity ratio of an X-ray spectrum of a standard specimen, including a silicon wafer, obtained at the time of obtaining the reference spectra, to an X-ray spectrum of the standard specimen, obtained immediately before matching.
Accordingly, it becomes possible to implement analysis of elements as to which spectra are overlapped with each other, and to obtain information on which element is contained in a foreign particle by checking the X-ray spectrum of the foreign particle, as well as a substrate under the foreign particle, in a region different in size, so that an electron microscope that is capable of analyzing elements and substances with high sensitivity and high precision can be provided. Further, since a difference in spectral sensitivity between instruments can be corrected by a correction curve using a standard specimen, it is sufficient to prepare one kind of database, which can be reinforced with a database acquired in another instrument without wasting the latter. Still further, even if the spectral sensitivity of the same instrument undergoes a change due to stains, etc. on the X-ray window, it is possible to effectively maintain matching with a database by acquiring a correction curve by use of a standard specimen.
(3) The electron microscope according to the invention may further comprise a memory for storing a plurality of X-ray spectra (sample spectra) at a plurality of observation points on the specimens, respectively, that have been obtained by the X-ray detector, and means for categorizing the plurality of the sample spectra into one or a plurality of groups of the sample spectra by matching them with each other and performing elemental analysis of one X-ray spectrum selected from the respective groups.
Furthermore, the electron microscope according to the invention may comprise a function of matching the sample spectra with each other for each of one or a plurality of X-ray energy regions set up so as to include respective peaks of the sample spectra.
Further, the electron microscope according to the invention may automatically categorize the plurality of the sample spectra by matching them with each other and perform elemental analysis of the plurality of the sample spectra as categorized.
Thus, it becomes possible to categorize, on the basis of representative spectra, without checking an X-ray spectrum obtained at every foreign particle point against a database every time the X-ray spectrum is obtained, so that the invention can provide an electron microscope that is capable of analyzing the elemental composition of respective foreign particles in a short time by matching the representative spectra only against the database, or performing qualitative analysis or quantitative analysis.